chemistry: atoms first julia burdge & jason overby chapter 6 representing molecules kent l....

Chemistry: Atoms FirstJulia Burdge & Jason Overby

Chapter 6

Representing Molecules

Kent L. McCorkle

Cosumnes River College

Sacramento, CA

Chemical Bonding I: Chemical Bonding I: Basic ConceptsBasic Concepts6

6.1 The Octet RuleLewis StructuresMultiple Bonds

6.2 Electronegativity and PolarityElectronegativityDipole Moment, Partial Charges and Percent Ionic Character

6.3 Drawing Lewis Structures6.4 Lewis Structures and Formal Charge6.5 Resonance6.6 Exceptions to the Octet Rule

Incomplete OctetsOdd Numbers of ElectronsExpanded Octets

According to the octet rule, atoms will lose, gain, or share electrons in order to achieve a noble gas electron configuration.

Only valence electrons contribute to bonding.

••••••

•• ••••

F• •F+ F F•• ••••••

•• ••••

Each F counts both shared electrons to “feel” as though it has a Ne

configuration.

F 1s22s22p5

valence electrons

The Octet RuleThe Octet Rule6.1

The Octet RuleThe Octet Rule

Only two valence electrons participate in the formation of the F2 bond.

Pairs of valence electrons not involved in bonding are called lone pairs.

F F•• ••••••

•• ••••

F F•• ••••••

•• ••••

F F••••••

•• ••••

or

Lewis StructuresLewis Structures

A Lewis structure is a representation of covalent bonding.

Shared electron pairs are shown either as dashes or as pairs of dots.

Lone pairs are shown as pairs of dots on individual atoms.

H O H••••

H O H••••

•• ••O with 8 e−

H with 2 e− H with 2 e−

Shared electrons shown as dashes (bonds)

Multiple BondsMultiple Bonds

In a single bond, atoms are held together by one electron pair.

In a double bond, atoms share two pairs of electrons.

H O H••••

C with 8 e−

O with 8 e−O with 8 e−

one shared pair of electrons results in a single bond

••O C O

•••• ••

••

••

••••

2 shared pairs of electrons result in double bonds

O C O

••

••

••

••= =

H O H••••

•• ••


A triple bond occurs when atoms are held together by three electron pairs.

each N has 8 e−

3 shared pairs of electrons result in a triple bond

••N N

•••••••• N N

•••• ≡


Bond length is defined as the distance between the nuclei of two covalently bonded atoms.


Multiple bonds are shorter than single bonds.

For a given pair of atoms:triple bonds are shorter than double bondsdouble bonds are shorter than single bonds

< <N≡N

110 pm

N=N

124 pm

N−N

147 pm


The shorter multiple bonds are also stronger than single bonds.

We quantify bond strength by measuring the quantity of energy required to break it.

H2(g) → H(g) + H(g) Bond energy = 436.4 kJ/mol

We will examine this in more detail in Chapter 10.

Electronegativity and PolarityElectronegativity and Polarity

There are two extremes in the spectrum of bonding:covalent bonds occur between atoms that share electronsionic bonds occur between a metal and a nonmetal and involve ions

Bonds that fall between these extremes are polar.

In polar covalent bonds, electrons are shared but not shared equally.

M:X

Pure covalent bondNeutral atoms held together by equally shared electrons

Mδ+Xδ−

Polar covalent bondPartially charged

atoms held together by unequally shared

electrons

M+X−

Ionic bondOppositely charged

ions held together by electrostatic attraction

6.2


Electron density maps show the distributions of charge.Electrons spend a lot of time in red and very little time in blue.

Electrons are shared equally

nonpolar covalent

Electrons are not shared equally and are more likely to be associated

with F

polar covalent

Electrons are not shared but rather transferred

from Na to F

ionic

ElectronegativityElectronegativity

Electronegativity is the ability of an atom in a compound to draw electrons to itself.


Electronegativity varies with atomic number.

ElectronegativityElectronegativity

There is no sharp distinction between nonpolar covalent and polar covalent or between polar covalent and ionic.

The following rules help distinguish among them:A bond between atoms whose electronegativites differ by less than 0.5 is general considered purely covalent or nonpolar.

A bond between atoms who’s electronegativies differ by the range of 0.5 to 2.0 is generally considered polar covalent.

A bond between atoms whose electronegativities differ by 2.0 or more is generally considered ionic.

Classify the following bonds as nonpolar, polar, or ionic: (a) the bond in ClF, (b)

the bond in CsBr, and (c) the carbon-carbon double bond in C2H4.

Worked Example 6.1

Solution (a)The difference between the electronegativies of F and Cl is 4.0 – 3.0 = 1.0, making the bond in ClF polar.(b)In CsBr, the difference is 2.8 – 0.7 = 2.1, making the bond ionic.(c)In C2H4, the two atoms are identical. (Not only are they the same element, but each C atom is bonded to two H atoms.) The carbon-carbon double bond is C2H4 is nonpolar.

Strategy Electronegativity values are: Cl (3.0), F (4.0), Cs (0.7), Br (2.8), C (2.5). Use this information to determine which bonds have identical, similar, and widely different electronegativities.

Think About It By convention, the difference in electronegativity is always calculated by subtracting the smaller number from the larger one, so the result is always positive.

Dipole Moment, Partial Charges, and Percent Ionic Dipole Moment, Partial Charges, and Percent Ionic CharacterCharacter

An arrow is used to indicate the direction of electron shift in polar covalent molecules.

The consequent charge separation can be represented as:Deltas (δ) denote a partial positive or negative charge.

Regions where electrons spend little time

Regions where electrons spend a lot of time

H−F••••

••

H−F••••

••

δ+ δ−

Dipole Moment, Partial Charges, and Percent Ionic CharacterDipole Moment, Partial Charges, and Percent Ionic Character

A quantitative measure of the polarity of a bond is its dipole moment (μ).

Q is the charge.

r is the distance between the charges.

μ is always positive and expressed in debye units (D).

μ = Q x r

1 D = 3.336×10−30 C∙m


A quantitative measure of the polarity of a bond is its dipole moment (μ).

μ = Q x r

Burns caused by hydrofluoric acid [HF(aq)] are unlike any other acid burns and present unique medical complications. HF solutions typically penetrate the skin and damage internal tissues, including bone, often with minimal surface damage. Less concentrated solutions actually can cause greater injury than more concentrated ones by penetrating more deeply before causing injury, thus delaying the onset of symptoms and preventing timely treatment. Determine the magnitude of the partial positive and partial negative charges in the HF molecule.

Worked Example 6.2

Strategy Solve for Q. Convert the resulting charge in coulombs to units of electronic charge. According to Table 6.2, μ = 1.82 D and r = 0.92 Å for HF. The dipole moment must be converted from debye to C∙m and the distance between ions must be converted to meters.

μ = 1.82 D ×3.336×10-30 C∙m

1 D= 6.07×10-30 C∙m

r = 0.92 Å ×1×10-10 m

1 Å= 9.2×10-11 m

Worked Example 6.2 (cont.)

Solution In coulombs:

In units of electronic charge:

Therefore, the partial charges in HF are +0.41 and -0.41 on H and F, respectively.

Q =6.07×10-30 C∙m

9.2×10-11 m= 6.598×10-20 C

μr

=

6.598×10-20 C ×1 e-

1.6022×10-19 C= 0.41 e-

H−F••••

••+0.41 -0.41

Think About It Calculated partial charges should always be less than 1. If a “partial” charge were 1 or greater, it would indicate that at least one electron had been transferred from one atom to the other. Remember that polar bonds involve unequal sharing of electrons, not a complete transfer of electrons.


Although the designations “covalent,” “polar covalent,” and “ionic” can be useful, sometimes chemists wish to describe and compare chemical bonds with more precision.

Comparing the calculated dipole moment with the measured values gives us a quantitative way to describe the nature of a bond using the term percent ionic character.

percent ionic character = ×100%μ (observed)

μ (calculated assuming discrete charges)


The figure below demonstrates the relationship between percent ionic character and the electronegativity difference in a heteronuclear diatomic molecule.

Using data from Table 6.2, calculate the percent ionic character of the bond in HI.

Worked Example 6.3

Strategy Calculate the dipole moment in HI assuming that the charges on H and I are +1 and -1, respectively; then calculate the percent ionic character. The magnitude of the charges must be expressed in coulombs (1 e- = 1.6022×10-19 C); the bond length (r) must be expressed as meters (1 Å = 1×10-10 m); and the calculated dipole moment should be expressed as debyes (1 D = 3.336×10-30 C∙m).

Setup From Table 6.2, the bond length in HI is 1.61 Å (1.61×10-10 m) and the measured dipole moment of HI is 0.44 D.


Solution The dipole we would expect if the magnitude of the charges were 1.6022×10-19 C is

Converting to debyes gives

The percent ionic character of the H–I bond is ×100% = 5.7%.

μ = Q × r = (1.6022×10-19 C) × (1.61×10-10 m) = 2.58×10-29 C∙m

2.58×10-29 C∙m ×1 D

3.336×10-30 C∙m= 7.73 D

0.44 D7.73 D

Think About It A purely covalent bond (in a homonuclear diatomic molecule such as H2) would have 0 percent ionic character. In theory, a purely ionic bond would be expected to have 100 percent ionic character, although no such bond is known.

Drawing Lewis StructuresDrawing Lewis Structures

Follow these steps when drawing Lewis structure for molecules and polyatomic ions.

1)Draw the skeletal structure of the compound. The least electronegative atom is usually the central atom. Draw a single covalent bond between the central atom and each of the surrounding atoms.

1)Count the total number of valence electrons present; add electrons for negative charges and subtract electrons for positive charges.

1)For each bond in the skeletal structure, subtract two electrons from the total valence electrons.

1)Use the remaining electrons to complete octets of the terminal atoms by placing pairs of electrons on each atom. Complete the octets of the most electronegative atom first.

1)Place any remaining electrons in pairs on the central atom.

1)If the central atom has fewer than eight electrons, move one or more pairs from the terminal atoms to form multiple bonds between the central atom and terminal atoms.

6.3

Drawing Lewis Structures

Draw the Lewis Structure for ClO3−.

Solution:Step 1 Draw the skeletal structure of the compound. The least electronegative atom is usually the central atom. Draw a single covalent bond between the central atom and each of the surrounding atoms.

Cl OO

O

chlorine’s electronegativity = 3.0oxygen’s electronegativity = 3.5



Solution:Step 2 Count the total number of valence electrons present; add electrons for negative charges and subtract electrons for positive charges.

Cl: 1 x 7 = 7 e−

O: 3 x 6 = 18 e−

Charge: 1 e−

Total: 26 valence e−


Cl OO

O



Solution:Step 4 Use the remaining electrons to complete octets of the terminal atoms by placing pairs of electrons on each atom. Complete the octets of the most electronegative atom first.

Cl OO

O

6 electrons used in single bonds

20 electrons must be placed in the structure starting with terminal atoms

The structure now has a total of 24 valence electrons of the available 26.

Cl OO

O



Solution:Step 5 Place any remaining electrons on the central atom.

Cl OO

O-

Cl OO

O

The structure now has a total of 24 valence electrons of the available 26.

2 e− remain to be added to the structure.

Lewis structures for polyatomic ions are enclosed by square brackets.



Solution:Step 6 If the central atom has fewer than eight electrons, move one or more pairs from the terminal atoms to form multiple bonds between the central atom and terminal atoms.

Cl OO

O-

All atoms already have an octet in this structure.

Draw the Lewis structure for carbon disulfide (CS2).

Worked Example 6.4

Setup Step 1: C and S have identical electronegativities. We will draw the skeletal structure with the unique atom, C, at the center.

Step 2: The total number of valence electrons is 16: 6 from each S atom and 4 from the C atom [2(6) + 4 = 16].

Step 3: Subtract 4 electrons to account for the bonds in the skeletal structure, leaving us 12 electrons to distribute.

Step 4: Distribute the 12 remaining electrons as 3 lone pairs on each S atom.

S C S

S C S

Draw the Lewis structure for carbon disulfide (CS2).


Setup Step 5: There are no electrons remaining after step 4, so step 5 does not apply.

Step 6: To complete carbon’s octet, use one lone pair from each S atom to make a double bond to the C atom.

S C S

Think About It Counting the total number of valence electrons should be relatively simple to do, but it is often done hastily and is therefore a potential source of error in this type of problem. Remember that the number of valence electrons for each element is equal to the group number of that element.

Solution

Lewis Structures and Formal ChargeLewis Structures and Formal Charge

Formal charge can be used to determine the most plausible Lewis Structure when more than one possibility exists for a compound.

To determine associated electrons:

1) All the atom’s nonbonding electrons are associated with the atom.

2) Half the atom’s bonding electrons are associated with the atom.

Formal charge = valence electrons – associated electrons

6.4


Determine the formal charges on each oxygen atom in the ozone molecule (O3).

O O O

••••

••

••= − ••••

e− associated with atom

Valence e−

Difference (formal charge)

6 6 6

6 5 7

0 +1 −1

4 unshared + 4 shared = 6 e−

2


2


2

The widespread use of fertilizers has resulted in the contamination of some groundwater with nitrates, which are potentially harmful. Nitrate toxicity is due

primarily to its conversion in the body to nitrite (NO2-), which interferes with the

ability of hemoglobin to transport oxygen. Determine the formal charges on each

atom in the nitrate ion (NO3-).

Worked Example 6.5

Strategy Follow the six steps to draw the Lewis structure of (NO3-). For each

atom, subtract the associated electrons from the valence electrons.

Setup

The N atom has five valence electrons and four associated electrons (one from each single bond and two from the double bond). Each singly bonded O atom has six valence electrons and seven associated electrons (six in three lone pairs and one from the single bond). The doubly bonded O atom has six valence electrons and six associated electrons (four in two lone pairs and two from the double bond.)

O N O

O


Solution

The formal charges are as follows: +1 (N atom), -1 (singly bonded O atoms), and 0 (doubly bonded O atom).

O N O

O

Think About It The sum of formal charges (+1) + (-1) + (-1) + (0) = -1 is equal to the overall charge on the nitrate ion.


When there is more than one possible structure, the best arrangement is determined by the following guidelines:

1) A Lewis structure in which all formal charges are zero is preferred.

2) Small formal charges are preferred to large formal charges.

3) Formal charges should be consistent with electronegativities.

O C O

••

••••

••= = O C O

•• ••

••••≡ −

0 0 0 +1 0 −1Formal charge

better structure (based on formal charge)


Based on formal charge, identify the best and the worst structures for the isocyanate ion below:

Solution:Step 1 Assign formal charges on each atom using the formula

4 5 6

−2 +1 0

Ve−

C N O-

C N O-

C N O-

Formal charge = valence electrons – associated electrons

C N O-

C N O-

C N O-

Ae−

FC

4 5 6 4 5 6

6 4 6 5 4 7 7 4 5

−1 +1 −1 −3 +1 +1


Based on formal charge, identify the best and the worst structures for the isocyanate ion below:

Solution:Step 2 Determine the best and worst structure

−2 +1 0

C N O-

C N O-

C N O-

FC −1 +1 −1 −3 +1 +1

Worst structure:

Large formal charges

Formal charges inconsistent with

electronegativities

Best structure:

Small formal charges

Formal charges more consistent with

electronegativities

Formaldehyde (CH2O), which can be used to preserve biological specimens, is

commonly sold as a 37 percent aqueous solution. Use formal charges to determine which skeletal arrangements of atoms shown here is the best choice for

the Lewis structure of CH2O.

Worked Example 6.6

Strategy The complete Lewis structures for the skeletons shown are:

H C O H H C H

O

H C O H H C H

O



In the structure on the left, the formal charges are as follows:

Both H atoms: 1 valence e- – 1 associated e- (from single bond) = 0

C atom: 4 valence e- – 5 associated e- (two in the lone pair, one from the single bond, and two from the double bond) = -1

O atom: 6 valence e- – 5 associated e- (two from the lone pair, one from the single bond, and two from the double bond) = +1

H C O H H C H

O

H C O H

0 0+1-1Formal charges



In the structure on the right, the formal charges are as follows:

Both H atoms: 1 valence e- – 1 associated e- (from single bond) = 0

C atom: 4 valence e- – 4 associated e- (one from each single bond, and two from the double bond) = 0

O atom: 6 valence e- – 6 associated e- (four from the two lone pairs and two from the double bond) = 0

H C O H H C H

O

Formal charges all zero

H C H

O


Solution Of the two possible arrangements, the structure on the left has an O atom with a positive formal charge, which is inconsistent with oxygen’s high electronegativity. Therefore, the structure on the right, in which both H atoms are attached directly to the C atoms and all atoms have a formal charge of zero, is the better choice for the Lewis structure of CH2O.

Think About It For a molecule, formal charges of zero are preferred. When there are nonzero formal charges, they should be consistent with the electronegativities of the atoms in the molecules. A positive formal charge on oxygen, for example, is inconsistent with oxygen’s high electronegativity.

Resonance

A resonance structure is one of two or more Lewis structures for a single molecule that cannot by represented accurately by only one Lewis structure.

Resonance structures are a human invention.

Resonance structures differ only in the positions of their electrons.

O O O

••

••

••

••= − ••••

O O O••

•••••• − = ••

••

6.5

High oil and gasoline prices have renewed interest in alternative methods of producing energy, including the “clean” burning of coal. Part of what makes “dirty” coal dirty is its high sulfur content. Burning dirty coal produces sulfur

dioxide (SO2), among other pollutants. Sulfur dioxide is oxidized in the

atmosphere to form sulfur trioxide (SO3), which subsequently combines with

water to produce sulfuric acid – a major component of acid rain. Draw all possible resonance structures of sulfur trioxide.

Worked Example 6.7

Strategy Following the steps for drawing Lewis structures, we determine that a correct Lewis structure for SO3 contains two sulfur-oxygen single bonds and one sulfur-oxygen double bond.

But the double bond can be put in any one of three positions in the molecule.

O S O

O


Solution

O S O

O

O S O

O

OSO

O

Think About It Always make sure that resonance structures differ only in the position of the electrons, not in the positions of the atoms.

Exceptions to the Octet RuleExceptions to the Octet Rule

Exceptions to the octet rule fall into three categories:

1) The central atom has fewer than eight electrons due to a shortage of electrons.

Elements in group 3A also tend to form compounds surrounded by fewer than eight electrons.

Boron, for example, reacts with halogens to form compounds of the general formula BX3 having six electrons around the boron atom.

H Be H only 4 total valence electrons in the system

6.6

2) The central atom has fewer than eight electrons due to an odd number of electrons.

Molecules with an odd number of electrons are sometimes referred to as free radicals.

O N O

••

••

••••= − ••

•

17 valence electrons in the system

Odd Numbers of ElectronsOdd Numbers of Electrons

Draw the Lewis structure of chlorine dioxide (ClO2).

Worked Example 6.8

Strategy The skeletal structure is

This puts the unique atom, Cl, in the center and puts the more electronegative O atoms in terminal positions.

There are a total of 19 valence electrons (7 from the Cl and 6 from each of the two O atoms). We subtract 4 electrons to account for the two bonds in the skeleton, leaving us with 15 electrons to distribute as follows: three lone pairs on each O atom, one lone pair on the Cl atom, and the last remaining electron also on the Cl atom.

Cl OO

Cl OO

Solution

Think About It ClO2 is used primarily to bleach wood pulp in the manufacture of paper, but it is also used to bleach flour, disinfect drinking water, and deodorize certain industrial facilities. Recently, it has been used to eradicate the toxic mold in homes in New Orleans that were damaged by the devastating floodwaters of Hurricane Katrina in 2005.

Expanded OctetsExpanded Octets

3) The central atom has more than eight electrons.

Atoms in and beyond the third period can have more than eight valence electrons.

In addition to the 3s and 3p orbitals, elements in the third period also have 3d orbitals that can be used in bonding.

SF

F F

F

F

F

Sulfur has 6 bonds corresponding to 12 electrons

Worked Example 6.9

Strategy The skeletal structure is

There are a total of 24 valence electrons (3 from the B and 7 from each of the three I atoms). We subtract 6 electrons to account for the three bonds in the skeleton, leaving 18 electrons to distribute in lone pairs on each I atom.

Solution

Draw the Lewis structure of boron triiodide (BI3).

B

I

II

B

I

II


Think About It Boron is one of the elements that does not always follow the octet rule. Like BF3, however, BI3 can be drawn with a double bond in order to satisfy the octet of boron. This gives rise to a total of four resonance structures.

B

I

II B

I

II B

I

I IB

I

II

Worked Example 6.10

Strategy The skeletal structure already has more than an octet around the As atom.

There are 40 total valence electrons [5 from As (Group 5A) and 7 from each of the five F atoms (Group 7A)]. We subtract 10 electrons to account for the five bonds in the skeleton, leaving 30 to be distributed. Next, we place three lone pairs on each F atom, thereby completing all their octets and using up all the electrons.Solution

Draw the Lewis structure of arsenic pentafluoride (AsF5).

As

F

F

F

F

F

As

F

F

F

F

F

Think About It Always make sure that the number of electrons represented in your final Lewis structure matches the total number of valence electrons you are supposed to have.

Worked Example 6.11

Strategy Begin by drawing the skeletal structure and counting the total number of valence electrons. Use the steps outlined in Section 6.4 to draw the first structure and reposition one or more lone pairs to adjust the formal charges for the second structure.

Note that each hydrogen in an oxoacid is attached to an oxygen atom, and not directly to the central atom. The total number of valence electrons is 26 (6 from S, 6 from each O, and 1 from each H).

Draw two resonance structures for sulfurous acid (H2SO3): one that obeys the

octet rule for the central atom, and one that minimizes the formal charges. Determine the formal charges on each atom in both structures.

H O S O H

O


Solution Following the steps we get the first structure:

From the top O atom, which has three lone pairs, we reposition one lone pair to create a double bond between O and S to get the second structure.

Incorporating the double bond results in every atom having a formal charge of zero.

H O S O H

O

+1

-1

H O S O H

O

Think About It In some species, such as the sulfate ion, it is possible to incorporate too many double bonds. Structures with three and four double bonds to sulfur would give formal charges on S and O that are inconsistent with the electronegativities of these elements. In general, if you are trying to minimize formal charges by expanding the central atom’s octet, only add enough double bonds to make the formal charge on the central atom zero.

Worked Example 6.12

Strategy Follow the steps for drawing Lewis structures. The skeletal structure is

There are 36 total valence electrons (8 from Xe and 7 from each of the four F atoms). We subtract 8 electrons to account for the bonds in the skeleton, leaving 28 to distribute. We first complete the octets of all four F atoms. When this is done, 4 electrons remain, so we place two lone pairs on the Xe atom.Solution

Draw the Lewis structure of xenon tetrafluoride (XeF4).

F Xe F

F

F

F Xe F

F

F

Think About It Atoms beyond the second period can accommodate more than an octet of electrons, whether those electrons are used in bonds or reside on the atoms as lone pairs.

Chapter Summary: Key PointsChapter Summary: Key Points6

Lewis StructuresOctet RuleBond OrderElectronegativity and Polarity Dipole Moment Percent Ionic CharacterDrawing Lewis StructuresFormal ChargeResonance StructuresExceptions to the Octet Rule Incomplete Octets Odd Numbers of Electrons Expanded Octets

• Draw Lewis Structures for the following substances:

• CO2

• CO• HCN• SO3

2-

Group Quiz #10

6161

• Draw all possible Lewis Structures for:

• POCl3• BF3

• NO2-

• PF5

• SO3

Group Quiz #11

chemistry: atoms first julia burdge & jason overby chapter 6 representing molecules kent l....

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multiple bonds multiple

shared pairs of electrons

shared electrons

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pairs of valence electrons

triple bonds

dashes bonds